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Just picked up an '89 325i a few weeks ago, and been baffled over this issue as it's almost impossible to recreate...it just happens randomly. On a cold start on warm days (usually after the car's been in the sun for a little while), sometimes (maybe 20% of the time) there's a seriously loud squeal that lasts maybe half a second when the engine catches and revs up. It immediately goes away and there are no issues/noises after that.
It sounds like a belt (rubber slipping on metal).
Does anyone know what it could be? I'm just baffled because i can't get it to happen again when i try. It seems just to do it randomly during those warmer days. Just wondering if it could be pointing to a more serious issue.
Any help greatly appreciated.
Thanks!

*edit: i'll try tightening all the aux belts this week and report back

Belts usually don't last long running the water pump, fan, and the alternator. I would head over to BMW and pick up a new Continental belt. This should solve your problem as long as you don't over tighten it.

Do you have air conditioning?
Do you leave the AC switch on when you shut down? The compressor will take a tremendous load on start, but I'd expect it to take a minute or two to load up, not start squealing right away on start.

Belt condition and tension come before fancy trouble-shooting. Good luck!

Not your problem by the sound (Harh!) of it, but I've got a mis-aligned PS pump pulley and it makes the belt squeal on and off as the idle speed varies.
Idle speed varies because vacuum leak, so my car sounds like a small dog barking.
Very embarrassing.

Not your problem by the sound (Harh!) of it, but I've got a mis-aligned PS pump pulley and it makes the belt squeal on and off as the idle speed varies.
Idle speed varies because vacuum leak, so my car sounds like a small dog barking.
Very embarrassing.

Kind of a relevant question actually - my PS makes a "soft" grinding noise when i'm on full look during parking etc. Is this normal? Fluid level is fine.